Nowadays, when narrow-band radio signals are used in low-power wide-area network networks (LPWAN), with the signal bandwidth of 500 Hz and less, an inaccuracy in generating carrier frequency by frequency synthesizers may result in occurrence of problems associated with misalignment of the frequencies of the transmitter and the receiver. For narrow-band radio signals transmitted from a subscriber device to the hub, this problem can be resolved by processing by the hub of a wide frequency band which is a priori larger than any misalignment of the frequencies of the transmitter of the subscriber device and the receiver of the hub. In the reverse direction from the hub to the subscriber device, radio signals are as a general rule received using off-the-shelf transceivers that process a narrow signal band which corresponds to the band of the expected signal without taking into account any possible discrepancies of the frequencies of the receiver and the transmitter. In this case, the carrier frequency of the signal in the radio signal should match exactly the passband of the input reception filters of the subscriber device for the successful reception of the radio signal. For example, for signals with a 200 Hz band, the alignment accuracy should be in the order of several tens of Hz.
There known LPWAN standards, such as LoRa™  SigFox™ standards. Devices that operate in accordance with the LoRa™ standard use signals with a spectrum extension. A drawback of that solution is a reduced spectral efficiency of the data network. To transmit messages from hubs to subscriber devices, SigFox™ uses signals with a bandwidth sufficient not to need the application of this invention. A drawback of that solution is a comparative reduction of the communication distance range during transmission from the hub to subscriber devices. Thus, the existing systems do not use a narrow-band modulation for downlink transmission of data.
U.S. Pat. No. 9,252,998 B2 entitled “Radio to detect and compensate for frequency misalignment” (patent holder: Itron Inc, publication date: Feb. 2, 2016 discloses a radio receiver which contains a radio frequency (RF) subsystem; a digital subsystem to obtain an input from the RF subsystem; a means to assess the frequency error for the obtained samples from the digital subsystem and to assess the shift of the frequency of the received signal from multiple received signals, in which case the tool for assessing the frequency error measures the frequency deviation according to the packet preamble length, and multiple received signals are associated with multiple various channel bands; as well as a transmitter to transmit data at the frequency which is based at last partially on the assessed frequency shift.